Lucian Patrick Crosthwaite

An extragalactic supernebula confined by gravity.

Little is known about the origins of globular clusters, which contain hundreds of thousands of stars in a volume only a few light years across. Radiation pressure and winds from luminous young stars should disperse the star-forming gas and disrupt the formation of the cluster. Globular clusters in our Galaxy cannot provide answers; they are billions of years old. Here we report the measurement of infrared hydrogen recombination lines from a young, forming super star cluster in the dwarf galaxy NGC5253. The lines arise in gas heated by a cluster of about one million stars, including 4,000–6,000 massive, hot ‘O’ stars. It is so young that it is still enshrouded in gas and dust, hidden from optical view. The gases within the cluster seem bound by gravity, which may explain why the windy and luminous O stars have not yet blown away those gases. Young clusters in ‘starbursting’ galaxies in the local and distant Universe may also be gravitationally confined and cloaked from view.

RADIO-INFRARED SUPERNEBULAE IN II Zw 40

We report subarcsecond-resolution Very Large Array and Keck mid-infrared imaging of the dwarf starburst galaxy II Zw 40. II Zw 40 contains a bright compact thermal radio and infrared source with all the characteristics of a collection of dense H ii regions ionized by at least 14,000 O stars. The supernebula is revealed to consist of multiple sources within an envelope of weaker emission. The radio emission is dominated by free-free emission at 2 cm, and the spectrum of this emission appears to be rising. This suggests that the free-free emission is optically thick at 2 cm and that the individual H ii regions are � 1 pc in size. This complex of ‘‘ supernebulae ’’ dominates the total infrared luminosity of II Zw 40, although the radio source is less than � 150 pc in diameter. Multiple super–star clusters appear to be forming here, the much larger analogs of large Galactic H ii region complexes.

The Large‐Scale Distribution and Properties of Carbon Monoxide in a Sample of Nearby Spiral Galaxies

Galaxies are defined by a cumulative history of converting of gas into stars, and molecular gas (H2) is the fuel for this star formation. Because H2 cannot be observed directly, we observe the “tracer” CO and derive the H2 mass indirectly via a “conversion” factor. To move beyond global correlations and the limitations of undersampled, limited-coverage maps, this study has used the NRAO 12 m telescope and its “on-the-fly” observing mode to efficiently and reliably map cold CO emission over very large areas in five spiral galaxies. The galaxies were selected based on previous CO detections, large angular size, low inclination, and morphology to form an illustrative sample ranging from big to small, starburst to nonstarburst, and actively interacting to passively quiescent. CO was mapped in roughly regions covering the optical disks of the selected ′ ′ 15 # 15 galaxies. This experiment has the following major results: (1) We now have large-scale, fully sampled maps of CO in IC 342, M83, NGC 1097, NGC 4736, and NGC 6946 (Fig. 1). The first largescale, total neutral gas (H2 H i) surface density maps Sgas of these galaxies are now available. (2) The CO maps confirm the correlation with the optical disks in overall extent and gross morphology, although significant morphological differences exist. Interarm molecular gas is common in the sample. In the case of NGC 1097, interarm regions in the grand-design pattern,